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MICROSTRUCTURE AND PROPERTIES OF MECHANICAL ALLOYED AND EQUAL CHANNEL ANGULAR EXTRUDED TUNGSTEN CARBIDE

A Thesis Presented to The Faculty o f the College o f Graduate Studies Lamar University

In Partial Fulfillment o f the Requirements for the Degree Doctor o f Engineering by Kannan Ramakrishnan

May 2001

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UMI Number: 3000459

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© 2001 by Kannan Ramakrishnan No part of this work can be reproduced without permission except as indicated by the “Fair Use” clause o f the copyright law. Passages, images, or ideas taken from this work must be properly credited in any written or published materiais.

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MICROSTRUCTURE AND PROPERTIES OF MECHANICAL ALLOYED AND EQUAL CHANNEL ANGILAR EXTRUDED TUNGSTEN CARBIDE KANNAN RAMAKRISHNAN

iproved:

Malur N. Snmvasan , Supervising Professor

Victor A. Zaloom Committee Member

James L. Thomas Committee Member

Paul R. Corder ittee Member

Hsmg-wei Chu Committee Member

Malur N. Srinivasan Chair, Department o f Mechanical Engineering

Jack R. Hopper / ff DeaiLCollege o f Engineering

________ ^Jam es W. Westgate Interim Associate Vice President for Research and Dean o f Graduate Studies

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ABSTRACT Tungsten carbide-cobalt (WC-Co) materials are widely used today in the area of heavy duty machining (tool bits), in cold-forming processes (as die materials) and in the oil field industry (drill bits). Limited ductility is exhibited by conventionally produced WC-Co parts because o f the presence o f a high volume fraction o f coarse WC particles in a soft cobalt matrix. Recent investigations indicate that the nanostructured WC-Co parts attain twice the hardness values to those corresponding parts with a coarse microstructure. This implies substantially extended wear and tear resistance o f WC-Co parts that have nanocrystalline microstructure. The focus o f this research is to develop a process to make nanostructured WC-Co parts in bulk form. This involves: 1) the use o f mechanical alloying (MA) to produce nanocrystalline tungsten carbide powder containing uniformly distributed cobalt (binder), and 2) the preservation o f the fine scale structure and composition in the parts, produced from this powder aggregate using the equal channel angular extrusion process (ECAE). An experimental plan based on a 23 factorial experimental design was used to determine the influence (effect) o f mechanical alloying parameters, namely, milling time, milling speed and ball to powder ratio, on the microhardness o f bulk processed WC-Co samples. The significance o f these variables was examined with respect to the average microhardness o f the extruded and annealed tungsten carbide compact. The combination of milling variables that was most beneficial and detrimental to the formation o f nanocrystalline compacts was established.

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X-Ray diffraction and scanning electron microscope (SEM) analysis to determine composition and grain size were also carried out to give a better insight to the formation of nanostructured WC-Co materials. This would facilitate the development o f the technology to produce superior quality tungsten carbide parts such as inserts, seats and dies required by the oil tool industry.

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ACKNOWLEDGEMENTS The author expresses his sincere regard for and gratitude to his advisor Dr. M. Srinivasan, for his patience, support, encouragement and guidance during the period of this study. The author is also grateful to the members o f the committee, Dr. V. Zaloom, Dr. P. Corder, Dr. J. Thomas and Dr. H. Chu for their erudite and valuable comments on and review o f this thesis. The support for this project provided by the Advanced Technology Program o f the State o f Texas is gratefully acknowledged. Dr. K. T. Hartwig o f Texas A&M University is sincerely thanked for allowing the use o f ECAE facilities for this research. The author would like to thank Dr. A. M. Clearfield and Ms. Zhike Wang o f Texas A&M University for their assistance in the X-ray diffraction analysis. The technical support provided by Mr. Robert Barber and Mr. A. Parasiris formerly o f Texas A&M University is also sincerely appreciated. The author wishes to thank his close friend Xhemal (Jim) Kaculi for his valuable help in conducting experiments, performing statistical analysis and collecting the pertinent literature. The author also extends his appreciation to Rajesh Bakhru for his assistance in fixing equipment problems. Special thanks are due to the author’s mother, father and sister for their understanding and encouragement in this study. It gives immense pleasure to the author to extend his gratitude to his close family o f friends: Cherian Thomas, Samir Ajit Patil, Ali Mehran Shahhosseini and Rahul Padmakar Mangire for their friendly support and advice and for making this project an interesting joy ride. iii

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TABLE OF CONTENTS LIST OF TABLES.................................................................................................................. vi LIST OF FIG U RES..............................................................................................................viii Page CHAPTER 1.

Introduction.................................................................................................................................1 Problem Statement................................................................................................................ 4 Objective o f S tudy................................................................................................................ 5

2.

Literature Review......................................................................................................................6 Mechanical Alloying............................................................................................................. 6 Equal Channel Angular Extrusion..................................................................................... 11 Nanocrystalline M aterials...................................................................................................16 Milling Procedure................................................................................................................18 Modeling Studies................................................................................................................ 20

3.

Experimental Planning and Procedure..................................................................................24 Design o f Experiments........................................................................................................24 Experimental Procedure..................................................................................................... 28

4.

Experimental Results and Discussions.................................................................................33

5.

Conclusions.............................................................................................................................. 43

6.

Suggestions for Future Research...........................................................................................45

iv

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REFERENCES........................................................................................................................46 APPENDIX A ..........................................................................................................................49 APPENDIX B .......................................................................................................................... 59 APPENDIX C .......................................................................................................................... 74 BIOGRAPHICAL N O T E ...................................................................................................... 81

v

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LIST OF TABLES Table

Page

1

Notations o f 23 factorial design........................................................................................26

2

Parameters o f mechanical alloying process....................................................................29

3

Average microhardness values o f WC-Co compacts for different alloying param eters............................................................................................ 34

4

Effects o f milling variables after annealing (using average microhardness values)........................................................................................................ 36

5

Average microhardness values and grain size values for Replicates 1 and 2 (after annealing)........................................................................... 39

6

Effects o f milling variables after annealing (using average microhardness and grain size values................................................................................ 40

7

Replicate 1: Vickers microhardness o f extruded WC-Cosamples at different points o f the material before annealing.............................................................50

8

Replicate 1: Vickers microhardness o f extruded WC-Cosamples at different points o f the material after annealing................................................................51

9

Replicate 2: Vickers microhardness o f extruded WC-Co samples at different points o f the material before annealing........................................................ 52

10 Replicate 2: Vickers microhardness o f extruded WC-Co samples at different points o f the material after annealing........................................................... 53

vi

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11 Best/Worst samples: Replicate 1 and 2 (beforeannealing)...........................................54 12 Best/Worst samples: Replicate 1 and 2 (after annealing)................................................ 55 13 Effects o f milling variables before annealing (using average microhardness values)......................................................................................................... 56 14 Modeling results........................................................... .......................................................57 15 Analysis o f Variance (ANOVA) o f microhardness values after annealing.........................................................................................................58

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LIST OF FIGURES Figure

Page

2.1

Evolution o f Mechanical Alloying Process.................................................................. 8

2.2

Balance between Solid State Welding and Fracture.................................................. 10

2.3

Concept o f Equal Channel Angular Extrusion............................................................ 12

2.4

Illustration o f the workpiece geometry during the ECAE process.......................... 13

2.5

Illustration o f the 1A and 2A ECAE extrusion pass.................................................. 14

2.6

Attritor used for Mechanical Alloying..........................................................................19

3.1

Schematic Representation of 23 Factorial Design.......................................................25

4.1

Intensity vs. Two Theta Plot for Replicate 1 - Sample 1 .........................................60

4.2

Intensity vs. Two Theta Plot for Replicate 1 - Sample 2 .........................................61

4.3

Intensity vs. Two Theta Plot for Replicate 1 - Sample 3 .........................................62

4.4

Intensity vs. Two Theta Plot for Replicate I - Sample 4 ......................................... 63

4.5

Intensity vs. Two Theta Plot for Replicate 1 - Sample 5 ......................................... 64

4.6

Intensity vs. Two Theta Plot for Replicate 1 - Sample 8 ......................................... 65

4.7

Intensity vs. Two Theta Plot for Replicate 2 - Sample 1 ......................................... 66

4.8

Intensity vs. Two Theta Plot for Replicate 2 - Sample 2 ..........................................67

4.9

Intensity vs. Two Theta Plot for Replicate 2 - Sample 3 ..........................................68

4.10 Intensity vs. Two Theta Plot for Replicate 2 - Sample 4 ..........................................69 4.11

Intensity vs. Two Theta Plot for Replicate 2 - Sample 5 .......................................... 70

4.12 Intensity vs. Two Theta Plot for Replicate 2 - Sample 6 .......................................... 71 4.13 Intensity vs. Two Theta Plot for Replicate 2 - Sample 7 .......................................... 72 viii

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4.14

Intensity vs. Two Theta Plot for Replicate 2 - Sample 8 ........................................ 73

4.15

SEM image o f Sample 7 - Replicate 2 at lOOx magnification.................................75

4.16

SEM image o f Sample 7 - Replicate 2 at 1,000x magnification............................. 76

4.17

SEM image o f Sample 7 - Replicate 2 at 10,000x magnification........................... 77

4.18

SEM image o f Sample 7 - Replicate 2 at 25,000x magnification........................... 78

4.19

SEM image o f Sample 7 - Replicate 2 at 50,000x magnification........................... 79

4.20

Plot o f Average Microhardess vs. Grain Size............................................................ 80

ix

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Ramakrishnan 1 CHAPTER 1 Introduction This research involves the use o f mechanical alloying (MA) and equal channel angular extrusion (ECAE) processes to produce tungsten carbide in bulk form. In what follows, each o f the constituents is described briefly. Tungsten Carbide (WC): The mechanical properties o f cemented tungsten carbides depend on binder content, average particle size, composition, and particle size distribution. It is by varying these parameters that the carbide producers formulate grades to fit specific applications. Other factors such as compacting pressure, sintering time and temperature, microstructure, porosity level, impurity concentration, and surface condition have a significant effect on properties o f the final product. Tungsten carbides themselves are too brittle to be used in industrial applications, but in combination with cobalt binder their properties can be improved. Binder content is very important in determining transverse rupture strength, hardness, impact strength and endurance limit. The transverse rupture strength, impact strength and endurance limit increase as the cobalt binder increases from 3-17%. With further increase o f cobalt up to 24%, the transverse rupture strength decreases. Compressive strength increases with decreasing cobalt content, but at a concentration o f 6% cobalt (as used in this research) and lower, there is a very small further increase. The carbon level in tungsten carbide is extremely important for good strength and wear properties o f the material. The strength of WC-Co materials increases with the increase o f carbon up to 6.2%. The hardness, density, and lattice parameter decrease as the carbon content decreases.

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Ramakrishnan 2 Cobalt bonded tungsten carbide (WC-Co) powders are used for the production o f near-net shape parts for a variety o f applications. WC-Co wear resistant parts are used in wire-forming and straightening, coil and spring machine parts, lathe centers and rests, valve seats, sandblast nozzles, thread guides, arbors, mandrels, micrometer jaws, snap and plug gauges, scale pivots, textile guides, bushings, etc. High and low speed cutting tools, dies, drawing tools, parts for cold-forming processes, items for the oil field industry and the items used in computer industry (thin hard metal wires, disc-type slitting knives and micro-drills) are some examples o f WC-Co applications.1 Previous studies indicate that the hardness o f WC-Co can be increased if constituent particle size is reduced and the tendency to (tool) chipping, cracking, and fracture can be greatly reduced if a nanocrystalline structure is obtained.2 Nanocrystalline materials can be produced by mechanical alloying process. So far, mechanical alloying has been widely applied to metals and their alloys, but only a few attempts have been made with hard alloys like WC-Co.3 Mechanical alloying is a method used to produce powders with a controlled fine microstructure. This is achieved by the repeated fracturing and solid state welding o f a mixture o f powder particles in a highenergy ball mill. For the formation o f alloys or compounds, the process requires at least one fairly ductile constituent to act as a host or a binder. The other constituents can consist o f ductile metals, brittle metals, intermetallic compounds, nonmetals, and refractory compounds. Mechanical Alloying (MA): The process o f developing a nanostructure may be purely mechanical or it may also involve chemical reactions between the constituents, as is the

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Ramakrishnan 3 case for the synthesis o f amorphous alloy powders.4 Mechanical alloying processes are normally carried out in high-energy ball mills, such as vibratory mills, ball mills, or large conventional ball mills. The process employs a mixture o f powders and grinding media (balls), within a wide range o f ball to powder ratios. In this investigation, the type o f mill used is a high-energy ball mill commonly referred to as an attritor. Equal Channel Angular Extrusion (ECAE): The equal channel angular extrusion method for imposing large plastic strains to materials was developed in the former USSR and recently introduced in United States. This method offers many advantages over conventional methods used for plastic deformation. The ECAE method can be used successfully for the production o f WC-Co cermets. Cermets are ceramic-metal systems that do not deform plastically. The ECAE materials processing method has the following advantages5: a) uniformly developed structural properties in the work-piece, b) application o f large amounts o f deformation (a benefit o f conventional extrusion) with no reduction in work-piece area, c) relatively low pressures, d) no special complex apparatus accommodations (standard presses are sufficient), and e) a wide operational temperature range.

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Ramakrishnan 4 Problem Statement Conventionally produced WC-Co parts have limited ductility because o f the presence of a high volume fraction o f coarse WC particles in a soft cobalt matrix. Experimental results have shown that the hardness o f WC-Co parts can be more than doubled when the grain, size o f the part is reduced from 500 nanometers (nm) to less than 200 nm.6 Scratch tests also indicate that the tendency to cracking can be dramatically reduced when the WC-Co has a nanostructure. This implies that the service life o f nanostructured WC-Co parts can be extended beyond those o f corresponding parts with a relatively coarse microstructure. This study focuses on the use o f mechanical alloying to produce nanocrystalline tungsten carbide powder containing a uniformly distributed cobalt (binder). Furthermore, the feasibility o f using ECAE process for preserving the nanostructure in bulk form is also examined.

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Ramakrishnan 5 Objective o f Study This study has the following objectives: 1) To use a mechanical alloying (MA) process to produce nanocrystalline tungsten carbide powder containing a uniformly distributed cobalt (binder), and to preserve the fine scale structure and composition in the parts produced from this powder aggregate using the equal channel angular extrusion process (ECAE). 2) To determine the influence o f three important mechanical alloying variables, milling time, milling speed, and powder to ball ratio, on the microhardness of WC-Co compacts after subjecting the mechanical alloyed powder to ECAE and annealing. 3) To quantify the effects o f milling time, milling speed and ball to powder ratio on the composition and grain size o f the compacts. This would facilitate optimization o f the “windows” o f operation (modeling) when WC-Co powder obtained by MA is produced in the final form using ECAE.

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Ramakrishnan 6 CHAPTER 2 Literature Review Mechanical Alloying (MA) By controlling the microstructures o f multiphase alloys, significant advances have been achieved in the properties o f engineered materials.7 Generally, microstructural refinements developed through processing lead to improvements in both strength and ductility. Thus, processing techniques that increase the range of attainable microstructures are fundamental to the development of new engineered materials. Mechanical alloying (MA) is one o f the processing routes (rapid solidification being the other) that has generated increased attention. The mechanical alloying process was developed 35 years ago for the production o f oxide dispersion strengthened superalloys.8 Initially, this process was used as a technique to alloy two or more metals that could not be alloyed with other methods present at that time. At present, mechanical alloying has changed its course. The main purpose o f this technique now is to produce crystalline and amorphous materials. This process uses highenergy ball mills for the processing o f elemental powders and binding elements that are present in the milling chamber. Mechanical alloying is a method used to produce composite powders with a controlled microstructure (nanocrystalline and amorphous). Although mechanical alloying has gained popularity, it is still quite a complicated process since many parameters are involved that have a significant effect on the final product. Some parameters involved in the process are milling time, milling speed, ball to powder ratio, size o f powder particles,

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Ramakrishnan 7 milling temperature, number o f balls, size o f balls, material o f balls, type o f milling equipment, and milling atmosphere. Mechanical alloying consists o f repeated welding and fracturing and can be considered to evolve in five stages, namely, initial period, period o f welding predominance, period o f equiaxed particle formation, start o f random welding orientation, and steady state processing.9 A schematic representation o f the evolution o f mechanical alloying process is given in Figure 2 .1.10 In a high-energy mill, the particles o f the metal powder are repeatedly flattened, fractured and re-welded. Every time two steel balls collide they trap powder particles between them. The force o f the impact deforms the particles and creates automatically clean new surfaces. When the clean surfaces come in contact, they weld together. Since such surfaces really oxidize, the milling operation is conducted in an atmosphere o f argon or an inert gas. At early stages in the process the metal powders are still rather soft, and the tendency for them to weld together into large particles predominates. As the process continues, the particles get harder, and their ability to withstand deformation without fracturing decreases. The larger particles are more likely to incorporate flaws and to break apart when they are struck by the steel balls. After a certain period o f time, the solid state welding and fracturing come into balance as shown in Figure 2.2.11The balance between solid state welding and fracturing is the key to the refinement o f internal structure of powders. This is an entirely solid state process. Experiments show that ball milling causes atomic disorder in intermetallic compounds.12 The employed milling conditions and alloy system manifest the ability for

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Ramakrishnan 8

100 urn Metal A

100 |im

Metal A Intermetallic Interdiffusion Dispersoid Dispersoids-^jjh

100 |im Metastable phase Dispersoids Intermetallic *-Dispersoids *“ Precipitate

Metal B Metal B 0.5|im

Concentration (of Metal B ... I.-5

Concentration of Metal A

The intermediate stage of MA Rapid Fracturing

Intermetallic

^T^y-Bqrjilibwm

0.5|Jm

The first stage of MA Intense Welding

• / *f

Dispersoids 0.5|im

The final stage of MA Moderate welding

Consolidation 100 (im

100 p.m

0.5nm

Completion of MA-steady state. Extremely deformed structure-lamellae no longer optically resolvable-metastable structure with dispersoids. Fine grained size and equilibrium distribution of dispersoids.

Figure 2.1 Evolution o f Mechanical Alloying Process Source: R. Sundaresan and F. H. Froes, “Mechanical Alloying,” Journal o f Metals (Aug. 1987): 22.

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Ramakrishnan 9 the powders to cold weld and fracture. It has been argued that this is the main source o f energy storage in ball milling, and if this energy storage makes the free energy higher than the free energy o f the amorphous phase, a crystalline to amorphous transition will take place. Thus, mechanical alloying offers greater latitude in controlling the microstructure than other non-equilibrium processing methods such as rapid solidification.

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Ramakrishnan 10

Fracture

20

30

40

60

80 100

400 600

PARTICLE SIZE (MICROMETERS)

Figure 2.2 Balance between Solid State Welding and Fracture Source: J. S. Benjamin, “Mechanical Alloying,” Scientific American 234 no. 5, (1976): 44.

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Ramakrishnan 11 Equal Channel Angular Extrusion fECAEI Recent investigations indicate that severe plastic deformation is an effective method for forming nanocrystalline materials. Different techniques have been used to introduce large quantities o f plastic strain into metals. Rolling is the most conventional technique, but higher strain levels have been achieved more recently.13 Equal channel angular extrusion (ECAE) is a preferred technique to produce severe plastic deformation. The uniqueness o f the ECAE method comes from its ability to produce intense and uniform plastic deformation caused by simple shear o f the material. The die system o f ECAE consists o f two channels with identical rectangular cross sections connected through the intersection by an adjustable angle. The extrusion is along the flow lines. During the extrusion process, the lubricated billet (slightly smaller than the die insert area) is placed in the first/upper channel and then extruded out from the second channel by applying a punch load. A thin layer in the billet located at the crossing plane o f the channels is deformed by simple shear at an angle (phi) from the extrusion axis, as shown in Figure 2.3.13 As extrusion occurs, layer after layer, all the material is uniformly subjected to deformation except at the two ends o f the billet. This is illustrated in Figure 2.414 and Figure 2.514. After extrusion, the entire billet is plastically deformed in nearly the same way by simple shear. The ECAE method has many technological benefits. Large sections o f material can be uniformly and intensively worked and a high level o f deformation can be reached after one pass. The level o f deformation becomes very large after multiple extrusion passes.

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Ramakrishnan 12

V

v$

Figure 2.3 Concept o f Equal Channel Angular Extrusion Source: Stephane M. Ferrasse, Vladimir K. Segal, K. T. Hartwig, and R. E. Goforth. “Microstructure and Properties o f Copper and Aluminum Alloy 3003 Heavely Worked by Equal Channel Angular Extrusion,” Metallurgical and Materials Transactions 28A, (Apr. 1997): 1047.

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Ramakrishnan 13

=45

A Schematic illustration of the ECAE process for a 90 0 die

Longttiidtnal direction 0 ) T ruuvcne direction (2) Flaw direction (3)

ShwjrPlarw

Lonottudnal Plane

Trorovene Plan®

RowPtane

fa)

(c )

Figure 2.4 Illustration o f the workpiece geometry during the ECAE process. Initial position (a), intermediate position (b) and one complete pass (c). Source: Anastasios Parasiris, “Consolidation o f WC-Co bv Simple Shear.” Masters thesis, Texas A&M University, 1999.

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Ramakrishnan 14

NOT TO SCALE

NO ROTATION

N=1

N=2

N=0

Figure 2.5 Illustration of the 1A and 2A ECAE extrusion pass (billet and element distortion is shown). The angle (0 :9 0 °, 22.30°, 13.20°) o f inclination to the longitudinal axis o f the billet element is indicated. Source: Anastasios Parasiris, “Consolidation o f WC-Co bv Simple Shear.” Masters thesis, Texas A&M University, 1999.

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Ramakrishnan 15 Also, modifying the plane and direction o f shear at each pass can develop a variety o f different microstructures, substructures, nanostructures, and textures. There are typically three specific cases or deformation routes.13 Route A: The billet orientation remains unchanged at each pass. Original equiaxed grains become lamellar because simple shear deformation increases gradually in the same direction. This is route exhibits limited porosity and higher compaction density. (Tassos). Route A was adopted for the present work. Route B: The material is deformed alternately in two orthogonal directions. The billet orientation at each pass is rotated 90 degrees in the clockwise and counterclockwise directions about the extrusion axis. Fibrous grain structures are created. Route C: The billet is rotated 180 degrees about its extrusion axis after each pass. As a result, structural elements are restored to their original shape after each even numbered pass. This procedure yields the formation o f a refined equiaxed grain structure. ECAE appears to be an extraordinary method to control the conditions o f deformation. Thus, ECAE can be used as a method to consolidate and preserve the nanostructured powders obtained from the mechanical alloying process.

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Ramakrishnan 16 Nanocrvstalline Materials Nanocrystalline materials, known also as nanostructured materials or nanomaterials, have been investigated in numerous studies in recent years.15 Nanocrystalline materials are materials possessing grain size (in nanometer-nm) on the order o f a billionth of a meter. A nanocrystalline material has grains on the order o f 1 - 100 nm. The grains are separated by high-angle grain or inter-phase boundaries. As such, they are inherently different from glasses (ordering on a scale o f < 2 nm) and conventional polycrystals (grain size o f > 1 micrometer). The novelty o f nanocrystalline materials is that they have a significant fraction o f the total atoms present at the grain boundaries, unlike their coarse-grained counterparts.15 Since the grain boundaries occupy a significant volume in their material, their physical properties greatly influence the overall material. The Hall-Petch relation that describes the scaling o f yield strength with grain size can be used as a basis for the excellent mechanical properties displayed by nanocrystalline materials. All nanostructured materials share three features: atomic domains (grains, layers or phases) spatially confined to less than 100 nm in at least one dimension, significant atom fractions associated with interfacial environments, and interactions between their constituent domains.16 Nanocrystalline materials manifest extremely fascinating and useful properties that can be exploited for a variety o f structural and nonstructural applications. Nanocrystalline materials are exceptionally strong, hard, and ductile at high temperatures. They are also wear resistant, erosion resistant, corrosion resistant, and chemically very active. They are

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Ramakrishnan 17 much more formable than their conventional, commercially available counterparts. Important keys to the future o f nanostructured materials will be (i) our ability to continue to significantly improve the properties o f materials by artificially structuring them on nanometer length scales and (ii) developing the methods for producing these materials in commercially viable quantities.17 In developing these new methods, it is imperative that an understanding o f the important role o f surface and interface chemistry in the assembly and resulting properties o f these materials be developed.

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Ramakrishnan 18 Milling Procedure In mechanical alloying, different types o f milling equipment are used: attrition mills, vibratory mills, and planetary mills. In principle, they all deliver mechanical energy via collisions between one or more grinding balls and the wall o f the milling chamber. However, the amount o f the energy transfer during individual collisions and how the energy is divided between compression and shear is equipment specific. For the purpose of this work, an attrition mill was used. The presence o f a large number o f balls and a rotating impeller with several arms make attrition mills much faster than conventional ball mills. An attritor as shown in Figure 2.618, is a vertically oriented ball mill that has paddles sticking in the charge that agitates it. A small electric motor rotates the internal shaft with paddles. A water-cooling jacket controls the temperature o f the process. All the milling was done under a protective atmosphere o f argon gas. Mixtures o f elemental powders of nominal composition were prepared and these powder mixtures, together with grinding balls (stainless steel balls 1/8 inch diameter) were sealed in the attritor chamber and then subjected to the mechanical alloying process. In order to have a protective atmosphere, a glove box was used to weigh the materials, to charge and discharge the attritor and to prepare samples for analysis. A transfer chamber enables samples to be passed between the laboratory and the glove box, while maintaining an established gas environment. This helps protect the user from sensitive materials like cobalt and also preserves materials from contamination by ambient air and moisture.

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Ramakrishnan 19

Drive Shaft

Input Material (Powders)

Rotating Impeller

Stationary Tank

Balls

Increasing Milling Time

Figure 2.6 Attritor used for Mechanical Alloying Source: H. J. Cui, R. E. Goforth, and K.T. Hartwig, “The Three-Dimensional Simulation of Flow Pattern in Equal-Channel Angular Extrusion,” Journal o f Metals (Aug. 1998): 23

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Ramakrishnan 20 Modeling Studies The MA process is complex and multifaceted, as it involves concepts o f mechanics, mechanical behavior, heat flow, thermodynamics and kinetics. Modeling approaches can be classified into two types: local and global.19 Local modeling describes the various effects (thermal and mechanical) and events (deformation, fracture and welding) that transpire when powder particles are entrapped between two colliding or sliding surfaces. Thus, local modeling is generic in the sense that parameters (relative impact velocity, angle o f impact between colliding work-pieces, charge ratio, etc.) affecting the various events are common to all devices. The values o f some o f the parameters (relative impact velocity) are specific to a particular type o f mill and its operations. Global modeling is device specific. This type o f modeling entails the study o f factors such as the distribution o f impact angles and the heterogeneity o f powder distribution within the mill. These factors which clearly differ from one type o f device to another. Successful modeling o f MA depends upon effective synthesis o f the local and global approaches. The whole process o f mechanical alloying can present the following obstacles:20 1)

Expensive, since powders with their relatively high initial cost are the starting materials.

2)

Milling o f hard materials causes mill wear and results in compositional alterations o f the powder.

3)

Nanocrystalline structures coarsen, and amorphous structures usually and perhaps always crystallize.

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Ramakrishnan 21 4)

Mechanical treatment is inefficient. Only a small fraction o f the mill energy is used in effecting the microstructural changes responsible for the intriguing powder properties.

In part, the low process efficiency has been an impetus for process modeling. Energy Transfer Evaluated By The Collision Model: If collision is the basic event by which power is transferred from the mill to the powder, then the main parameters to evaluate are28: a) the kinetic energy involved in each collision b) impact velocity and collision frequency c) power involved in the milling process a)

The Kinetic energy involved in each collision can be expressed by28: AE = K. (1/2) mb Vb2 where, mb is the mass o f the ball, and Vb is the relative impact velocity. Ka is a coefficient depending on the elasticity o f the collision: Ka=0 for elastic collision and K ,=l for perfect inelastic collision (balls covered with powder).

b)

The relative impact velocity o f a ball is given by20: Vb = (a/Rb)2 (Hv/p )1/2 where, a is the indentation radius, Rb is the ball radius, Hv is the hardness o f the lead balls, and p is the density of steel balls.

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Ramakrishnan 22 c)

The collision frequency o f the ball can be expressed by28

v = Kv co where, K v is a constant that depends on the mill geometry and co is the rotational speed o f the attritor. d)

The total collision frequency is given by28

vt = Kv © Nb where, Nb is the number o f balls in the mill chamber. e)

The power involved in a milling process is given by the intensive factor o f a single event, AE, multiplied by the number o f events per unit o f time (extensive factor), i.e28.: P = AE vt The values o f impact velocity and impact frequency were taken from published

literature.20 These values were established for various regions within the attritor at two specific speeds o f 250 rpm and 500 rpm. The values o f kinetic energy involved and power consumed during a single collision event are enumerated in APPENDIX A: Table 14. In the region o f impact and mixing, the energy and power consumption values are high (as expected) as this is the most active region in an attritor. Mechanical alloying is principally achieved by the combination o f impact and rolling/sliding events. Powder dead zone indicates the lack o f milling activity. The impact velocity and collision frequency are zero in this dead zone because the impeller arm is two ball diameters above the bottom of the attritor. Hence there is no power transmitted to the bottom layers. These results pertain to only two values o f milling speed. To make reasonable predictions from

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Ramakrishnan 23 the model, it is imperative to incorporate milling time and ball to powder ratio as well and this can be accomplished by using similarity principles (Buckingham Pi Theorem). Knowledge o f critical process variables provides an opportunity for the design o f improved milling devices and for reducing the empirical testing used for optimizing milling schedules. Models developed for complex processes cannot be expected to be absolutely precise. Rather, they are intended to identify important parameters, define the functional dependence o f the process output (e.g., density and grain size for hot isostatic processing) on process variables, and predict results with an acceptable level o f precision. One useful result o f such process modeling is considerable reduction in the empirical studies needed to refine a process into a useful engineering tool.21 In the broadest sense, modeling o f mechanical alloying seeks to define the important process parameters controlling the structure and the properties o f the resulting powders. Notwithstanding the extensive commercialization of the process, little has been published on the optimum conditions for the mechanical alloying and ECAE o f specific alloy systems, and the models for the process are only in the early stages o f development.7 Model development requires data on the evolution o f the morphology and structure o f the powder as a function o f attrition time for a given set o f powder/balls weight ratio, ball size and density, ball velocity, frequency, and distribution, and processing temperature. In the present case the model should also incorporate the variables involved in ECAE procedure. Although some data have been collected7, modeling is complicated because, in general, results on one mechanical alloying device do not scale to another.

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Ramakrishnan 24 CHAPTER 3 Experimental Planning and Procedure Design o f Experiments Experiments are used to study and learn things concerning a certain process or system. In this case we are interested to know how the mechanical alloying parameters (milling speed, milling time, and ball to powder ratio) affect the microhardness o f WC-Co final product. We use the design o f experiments to determine which o f these variables has the most significant effect on the microhardness. If, for example, we know that milling time has a negative effect, and, since we have control over this variable, we simply minimize milling time to minimize its negative effect on the product.22 If milling speed has a positive effect on the microhardness o f the product, we simply increase the number o f shaft rotations. But if the combination o f increase in milling speed and decrease in milling time favors the contamination o f elemental powders over which we have no control, we will end up with the wrong conclusion about the effect o f these variables. The main purpose o f factorial design is to study the interaction effects o f more than one factor involved in the process. A complete study o f a design, in which “k” factors are involved, requires 2k experiments to be performed and is called 2k factorial design.23 For the purpose o f this research 23 factorial design is used since there are three main factors (variables) that affect the output o f the experiment: milling time, milling speed, and ball to powder ratio. The eight different combinations are shown in Figure 3 .123 and in Table 1.

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Ramakrishnan 25

A = Milling Time B = Milling Speed C = Ball to Powder Ratio

be

abc

+ High c



ac

Factor C

+ H igh

Factor B -Low

- Low

- Low

+

+ High

Factor A

Figure 3.1 Schematic Representation o f 23 Factorial Design Source: Douglas C. Montgomery, Design and Analysis o f Experiments 3rd ed. (NewYork: John Wiley & Sons, 1991), 270.

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Ramakrishnan 26

Table 1 Notations o f 23 factorial design

Experiment Number

Milling Time A

Milling Speed B

Ball to Powder Ratio C

Micro­ hardness

1

low

low

low

(1)

2

high

low

low

a

3

low

high

low

b

4

low

low

high

c

5

high

high

low

ab

6

high

low

high

ac

7

low

high

high

be

8

high

high

high

abc

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Ramakrishnan 27 The average effect o f A, B, C, AB, AC, BC, ABC is expressed by these equations24: A = l/4n[a + ab + ac + abc - (1) - b - c - be] B = l/4n[b+ ab + be + abc - (1) - a - c - ac] C = l/4n[c + ac + be + abc - (1) - a - b - ab] AB = l/4n{[abc + ab + c + (1)] - [be + b + ac + a]} AC = l/4n[(l) - a + b - a b - c + a c - b c + abc] BC = l/4 n [(l) + a - b - ab - c - ac + be + abc] ABC = l/4n[abc - b c - a c + c - a b + b + a - (1)] Where: A is the effect o f milling time on microhardness B is the effect o f milling speed on microhardness C is the effect o f ball to powder weight ratio on microhardness AB, AC, BC, and ABC represent the effect o f the above interactions on microhardness n is the number of replications (1) is the microhardness for minimum values o f all three parameters a is the microhardness for maximum milling time b is the microhardness for maximum milling speed c is the microhardness for maximum ball to powder ratio ab is the microhardness for maximum milling time and milling speed be is the microhardness for maximum milling speed and ball to powder ratio abc is the microhardness for maximum milling time, milling speed, and ball to powder ratio.

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Ramakrishnan 28 Experimental Procedure During the implementation o f the experiments, a careful consideration was given to the different factors involved in the process. Some of the things considered were the type o f milling equipment, milling time, milling speed, milling atmosphere, milling temperature, powder composition, ball to powder ratio, contamination effects, the number and material o f grinding balls, and powder quantities. These different parameters have a direct effect on the microstructure and properties of the final product. Different conditions may produce entirely different results. For the purpose o f this project, three milling variables or parameters viz.: ball to powder weight ratio o f 10:1, milling speeds of 150 and 550 rotations per minute, and milling times o f 10 and 40 hours, were used in various desired combinations as listed in Table 2. Tool-quality tungsten carbide powder and the constituent for the blend (cobalt) was purchased in one lot to ensure consistency o f the composition. The grinding media (stainless steel balls 1/8 inch diameter), and the powder blends o f tungsten (99.5% pure, 112.7 grams), graphite (99.8% pure, 7.32 grams) and cobalt (99.8% pure, 7.32 grams) were subjected to mechanical alloying in an attritor (high-energy ball mill) chamber. All milling was done under a protective atmosphere of argon gas. Sixteen experiments (23 factorial experimental design - eight experiments in each replicate) were performed by changing the milling parameters: milling time, milling speed and ball to powder ratio. The composition however, was unaltered.

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Ramakrishnan 29 Table 2 Parameters o f mechanical alloying process

Experiment Number

Milling Time (Hours)

Milling Speed (Rpm)

Ball to Powder Weight Ratio

I

10

150

10:1

2

40

150

10:1

3

10

550

10:1

4

10

150

30:1

5

40

550

10:1

6

40

150

30:1

7

10

550

30:1

8

40

550

30:1

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Ramakrishnan 30 a) Extrusion Procedure: Powder samples pertaining to both the replicates were sealed by electron beam welding (at Ames DOE Labs-Iowa State University) in different holes in 316 stainless steel billets (lin x lin x 6-7 in). These billets were subjected to a onehour annealing process at 1200 °C, under a protective atmosphere and were then extruded using the equal channel angular extrusion process. Route A was used in the extrusion process. In Route A processing, the same orientation o f the shear plane and shear direction relative to the extrusion direction is maintained during all extrusions and promotes increased compaction.14 The billets were air quenched, measured and machined slightly, measured and coated (polymer resin), and then placed in the furnace for the second extrusion pass. b) Cutting Procedure: Two sets per replicate o f WC-Co samples (each set comprising eight different experiments) were cut from the billets and then polished per ASTM E-3.25 At least one inch at each end o f the extruded billets was removed from testing, since they were not uniformly deformed by ECAE. The sections that were cut from the extruded cans were from the middle o f the billet (and moving to either sides). Care was exercised in obtaining only the WC-Co samples and not the stainless steel material surrounding them. c) Polishing Procedure: The materials needed for polishing are 8 0 ,1 2 0 ,3 4 0 ,6 0 0 ,8 0 0 1200 and 1800-2400 SiC (use water as lubricant) polishing papers and diamond polishing paste (3 micrometer and 1/4 micrometer). The order o f polishing is from 80 grit and up for rough samples or from 340 grit and up for smooth samples (then down to 1/4

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Ramakrishnan 31 micrometer). The samples were ultrasonically cleaned for a few seconds to ensure that the polishing debris was cleaned o ff between all grinding and polishing steps. d) Annealing Procedure: The polished samples assigned for annealing were smooth on all sides. All the sample sides had at least a 600 grit polish. The side that was used for future testing (microhardness) had a V* micron finish. This was a necessary polishing preparation for the samples that were to be used for annealing. The reason for this procedure is that at high temperatures, (annealing done at 1400 °C) cobalt floats (partially melts) and deposits preferentially on sharp or valley surfaces. The surface irregularities act as sinks for cobalt and create cobalt lakes14(detrimental phases) that dry out different sample areas and make them brittle. One o f the sets was subjected to annealing under an argon atmosphere at 1400 °C with a heating rate o f 5 °C per minute, dwelling/annealing first at 150-270 °C for 10 minutes (hydrocarbons are eliminated at this stage), then at 1100 °C for 30 minutes (trapped gases escape and solid state sintering occurs) and then at 1400 °C for 60 minutes.14 These samples were later cooled at the rate o f 5 °C per minute to room temperature and then polished again using polishing paper o f 340 grit down to 1/4 micrometer. e) Microhardness Testing Procedure: The experimental plan was based on a 23 factorial experimental design that possesses the advantage o f quantifying the results with a relatively small number o f experiments resulting in vastly reduced project cost. Vickers microhardness values were determined, per ASTM E-384,26 for both replicates of WC-Co extruded compacts. Load o f 1kilogram and load time o f 15 seconds was used for the microhardness testing. These measurements were obtained using a Buehler Micronet II

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Ramakrishnan 32 digital microhardness tester with a diamond pyramid indentor. The average microhardness values were then incorporated in the design o f experiment equations to determine the influence (effects) o f mechanical alloying parameters, namely, milling time, milling speed and ball to powder ratio, on the microhardness o f WC-Co samples. The entire above-mentioned procedure was repeated to get another two sets o f compacts containing eight samples each. This was carried out as a second replication to satisfy the 23 design o f experimental procedure. The compacts were crushed to powder in a hammer bill and then subjected to X-ray diffraction for structure identification and particle size analysis. The instrument used was Seifert-Scintag PAD-V. Other relevant details are as follows: power source: Cu sealed tube radiation (Cu K-alpha = 1.54178 Angstrom) with Ni filter, V = 40KV, I = 30mA, Scanning: step size = 0.04 deg, fixed time = 1, slits = 1 deg, scanning speed = 1 deg/minute. Samples were ground into fine powder using a hammer mill and then loaded in a plastic holder. Grain size estimations were done using Scherrer’s equation27 and the effects o f the milling variables on the grain size o f the compacts were also tabulated. SEM analysis was also carried out on one o f the samples for microstructure and porosity evaluations. The objective was to obtain a better insight on the formation of nanostructured WC-Co materials from the effects o f milling time, milling speed, and ball to powder ratio on the composition and grain size o f the product will. The results would then facilitate optimization o f the windows o f operation (modeling) when WC-Co powder obtained by MA is produced in the final form using ECAE.

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Ramakrishnan 33 CHAPTER 4 Experimental Results and Discussions When the billets (Replicate I) were cut to analyze powders, it was found that the four circular holes at the ends o f the billets could not be distinctly seen. Instead, two big elliptic holes were present. Four circular holes could only be distinguished as the cuts were made towards the center o f the billet. This only shows that the ends o f the billet had not undergone uniform plastic deformation. This is the usual case in ECAE procedure. The billets in Replicate 2 however, displayed somewhat different results. Even when cuts were made towards the center of the billets, all four circular holes could not be observed at the same time. Only two circular holes (in one row or column depending on the viewing orientation) could be observed when cuts were made from one side o f the billet (and moving towards the center). The other two circular holes (in the second row or column) were distinguished when cuts were made from the other (opposite) side o f the billet and moving towards the center. Similar to Replicate 1, two big elliptical holes were observed at the ends o f the billets. The reason for this anomaly can be attributed to the fact that the blind holes in the billets (for Replicate 2) were not drilled in a linear direction, which would suggest a machining error rather than an extrusion process error. The individual Vickers microhardness values for replicates land 2, obtained at various areas/points on the WC-Co samples before and after annealing, are listed in APPENDIX A: Table 7 through Table 10. The average Vickers microhardness results obtained for the extruded WC-Co samples before and after annealing for two replications and for different milling variables are presented in Table 3.

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Ramakrishnan 34 Table 3 Average microhardness o f WC-Co compacts for different alloying parameters

Exp.

Ball to

No./

Milling

Milling

Desi­

Time

Speed

gn of

(Hours)

(Rpm)

Powder Weight Ratio

Expt.

Replicate No. 1

Replicate No. 2

(HV)

(HV)

(HV)

(HV)

Before

After

Before

After

Annea­

Annea­

Annea­

Annea­

ling

ling

ling

ling

1/(1)

10

150

10:1

453.60

660.08

494.34

544.29

2/a

40

150

10:1

319.40

745.75

279.44

416.10

3/b

10

550

10:1

386.00

777.66

496.55

567.98

4/c

10

150

30:1

235.40

696.33

513.55

528.52

5/ab

40

550

10:1

256.00

1050.83

341.75

706.10

6/ac

40

150

30:1

495.80

1105.30

871.72

964.29

7/bc

10

550

30:1

586.40

510.25

565.09

460.56

8/abc

40

550

30:1

267.50

536.16

438.48

461.23

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Ramakrishnan 35 It was noted that Experiment No. 5 showed a substantial increase in microhardness in both the replicates after annealing. This can be attributed to the imbalance between fracturing and welding. The interaction o f high speed and high ball to powder ratio produced the worst results as evident in Experiment No. 7 and Experiment No. 8 in both the replicates. Experiment No. 7 showed a decrease in the value o f microhardness after annealing. This was the worst combination and clearly shows that the mechanical alloying process was not carried out normally. It can also be noted that the highest average microhardness value o f the final product (after annealing) was achieved with milling time o f 40 hours, speed o f 150 rpm, ball to powder ratio o f 30:1, for 2 replicates. It was observed that the lowest average microhardness value o f the final product (after annealing) was achieved with milling time o f 10 hours, milling speed o f 550 rpm, and ball to powder ratio o f 30:1, for both the replicates. Also, as enumerated in APPENDIX A: Table 12, the three best samples: Sample Nos. 6 ,5 and 3 have the same respective combination o f milling factors in both the replicates (after annealing). The average microhardness values o f the samples o f both replicates (before annealing and after annealing) were substituted in the 23 factorial design equations. The independent effects of the three factors and their interactions were also studied by combining the two replicates, i.e. taking the average o f the values o f replicate no. 1 and 2, and then comparing these results with the results o f replicate no. 1 and no. 2 alone respectively. The results obtained for the samples after annealing are enumerated in Table 4 below. The results obtained for the samples before annealing are listed in APPENDIX A: Table 13.

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Ramakrishnan 36 Table 4 Effects o f milling variables after annealing (using average micohardness values) Average, of Replicate No. 1 & No. 2 Replicate No. 1

Replicate No. 2 (or number of replicates n = 2) Milling time,

A = 111.59

Milling time,

Milling speed, B = -82.39

Milling speed,

B = -64.33

Milling speed, B = -73.76

Ball/Pow. ratio, C = -95.82

Ball/Pow. ratio, C = 45.03

Ball/Pow. ratio, C = -25.39

Time & speed,

Time & speed,

Time & speed,

Milling time,

A= 197.68

A = 154.64

AB = -45.17

AB = -42.20

AB = -48.15 Time and

Time and

Time and

ball/pow. ratio, AC = 19.76

ball/pow. ratio, AC = 106.63

ball/pow. ratio, AC = 63.19

Speed and

Speed and

Speed and

ball/pow.ratio,

ball/pow. ratio,

ball/pow. ratio,

All 3 factors, ABC = -143.39

BC = -258.20

BC = -221.18

BC = -295.22

All 3 factors,

All 3 factors, ABC=-175.35

ABC = -160.15

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Ramakrishnan 37 The effect o f milling time is positive and agrees with the values obtained individually in both replicates. Milling time is the only factor that significantly and positively affects the process. This fact is established by the fact that the two best samples, Sample No. 6 and Sample No. 5 as shown in Table 3, in both the replicates have high milling time. There is slight increase in the negative effect o f milling speed on the process after annealing. The effect o f ball to powder ratio factor departs (positively) in its effect on the process from Replicate No. 1 This again is a preliminary judgm ent disregarding experimental errors and interaction effects. The interaction effect o f milling time and milling speed has a moderate negative effect on the milling process in both the replicates. The combination o f high milling time and high ball to powder ratio shows a very high positive value as established by the fact that the best sample (sample 6) in both the replicates was comprised o f this combination. The combination o f high milling speed and high ball to powder ratio was most detrimental to the process. The fact that the combination o f high milling speed and high ball to powder ratio produces the worst effects and samples is evident in the high value o f -258.20 units after annealing. There is a drastic difference between the values before and after annealing. The worst samples for both the replicates were Sample No. 7 and Sample No. 8. In case o f Sample No. 7, there was a decrease in the value o f microhardness after annealing. This could be either due to grain coarsening after ECAE or due to incomplete formation of nanocrystalline tungsten carbide. The combination o f all three primary factors gave a very high average effect o f -160.15 units. This negative interaction is

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Ramakrishnan 38 clearly evident in the low microhardness value o f Sample No. 8 in both the replicates. These conclusions are based on the microhardness o f the compacts. The samples were crushed to powder using a hammer mill and then subjected to X-ray diffraction analysis. Two o f the samples (sample nos. 6 and 7) pertaining to replicate no. 1 crumbled into a mass o f plastic and fine chunks o f material as they were being removed from the metallurgical mounts. Hence, these two samples could not be used for X-ray analysis. Fourteen diffraction patterns were obtained (as shown in APPENDIX B: Figures 4.1 to 4.14) and were analyzed for grain size using Scherrer’s equation27 which is as follows: D = K X / (P cos0) where, D = Crystallite size in Angstrom units (normal to diffracting planes). K = 0.89 = Scherrer constant (crystallite shape constant) k = X-ray wavelength (1.5418 Angstrom) used in the experiment P = Observed diffraction peak breadth at half-maximum intensity (full-width-halfmaximum) measured in radians 0=

Bragg angle

The K-alpha separation correction factors were also applied for calculating the grain size27. Table 5 lists the average microhardness values and the values o f grain size in nanometers for replicate nos. 1 and 2 after annealing. The effects o f milling time, milling speed, and ball to powder ratio on microhardness and the grain size o f the samples after annealing (replicate 2 only) are listed in Table 6.

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Ramakrishnan 39 Table 5 Average microhardness values and grain size values for Replicates 1 and 2 (after annealing)

Exp.

Replicate No. 1

Replicate No. 2

Ball to No./

Milling

Milling

Design

Time

Speed

Powder (HV)

(HV) Weight

of

(His.)

(Rpm.)

After

Grain

After

Grain

Annea­

Size (nm)

Annea­

Size (nm)

Ratio Expt.

ling

ling 1/(1)

10

150

10:1

660

51

544

33

2/a

40

150

10:1

743

30

416

49

3/b

10

550

10:1

778

66

568

28

4/c

10

150

30:1

696

49

529

64

5/ab

40

550

10:1

1051

no

706

45

6/ac

40

150

30:1

1105



964

54

7/bc

10

550

30:1

510



461

59

8/abc

40

550

30:1

536

461

73

65

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Ramakrishnan 40 Table 6 Effects o f milling variables after annealing (using average microhardness and grain size,, values) Effect o f Microhardenss Milling time,

A = 111.59

Effect o f Grain Size Milling time,

A = 9.04

Milling speed,

B = -64.33

Milling speed,

B = 1.08

Ball/Pow. ratio,

C = 45.03

Ball/Pow. ratio,

C = 23.91

Time & speed,

AB = -42.20

Time & speed,

AB = 6.42

Time and

Time and ball/pow. ratio,

AC = 106.63

Speed and

ball/pow. ratio,

AC = -7.03

Speed and

ball/pow. ratio,

BC = -221.18

ball/pow. ratio,

BC = 5.69

All 3 factors,

ABC = -175.35

All 3 factors,

ABC = 5.60

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Ramakrishnan 41 The effects o f the variables for replicate I could not be tabulated as two o f the samples (samples 6 and 7) were destroyed during analysis. As shown by the grain size values, nanosized grains are obtained in all the samples (both the replicates) after the samples are subjected to MA and ECAE procedures. It should be noted that microhardness testing was done on solid samples. X-ray analysis o f all the available samples (and SEM analysis o f one sample) was done using powder form o f the samples. Since the compacts were hammer milled to powder form, it is highly possible that all the grains did not fracture exactly along the grain boundaries. This may explain the lack o f correlation (coefficient of correlation r = -0.136) between the average microhardness values and the grain size values for all the samples in replicate 2. This graph is shown in APPENDIX C: Figure 4.20. The negative value o f r implies that there is an increase in microhardness with the decrease in grain size. Analysis o f variance (ANOVA) was carried out for both the replicates to further establish the significance o f the effects o f the variables involved. The results are shown in Appendix A: Table 15. The combination o f high milling speed and high ball to powder ration was most significant at 95% and 99% confidence levels. The worst sample (Sample 7) belongs to this category. This combination was most detrimental to the process in terms o f average microhardness. High milling time and the combination o f all three primary variables (all at higher level) were significant at 90% confidence level. ANOVA was carried out on all the values o f the samples that were obtained after annealing.

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Ramakrishnan 42 SEM analysis was carried out on Sample 7 (Replicate 2 - after annealing). SEM photographs were obtained at magnifications o f lOOx, lOOOx, 10,000x, 25,000x and 50,000x (as shown in APPENDIX C: Figures 4.15 to 4.19). The first three magnifications show clusters o f grains. The magnification o f 50,000x could not be used for lack o f clarity. Grain size estimation was done using the 25,000x magnification image. The image shows a cluster o f grains rather than an individual grain for reasons mentioned in the precious paragraph. Also, due to the two-dimensional nature o f the images, the grain size values estimated from the SEM images cannot be as accurate as the values obtained from X-ray diffraction patterns. Hence, the grain size values obtained from Scherrer’s equation are reported here.

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Ramakrishnan 43 CHAPTER 5 Conclusions Based on the above results, the following conclusions may be drawn for the experiments performed. 1.

The effect o f milling time alone is positive and agrees with the values obtained individually in both replicates after annealing. Milling time is the only factor that significantly and positively affects the process. The significance o f this variable is also established by the analysis o f variance. This fact is established by the fact that the two best samples, Sample No. 6 and Sample No. 5 as shown in Table 3, in both the replicates have high milling time.

2.

The interaction effect o f high milling time and high ball to powder ratio shows a very high positive value for both the replicates (after annealing). This conclusion is supported by the fact that the best sample in both the replicates (Sample 6) had the combination o f a high milling time o f 40 hours, high ball to powder ration o f 40:1 and low milling speed o f 150 rpm. The individual and average Vickers microhardness values o f sample 6 for both the replicates supported this fact. This is the most favored combination for obtaining the best results in terms o f average microhardness values o f the compacts subjected to MA and ECAE.

3.

The interaction effect o f milling time and milling speed has a moderate negative effect on the milling process in both the replicates. The interaction o f high speed and high ball to powder ratio produced the worst results as evident in Experiment No. 7 and Experiment No. 8 in both the replicates. The significance o f this

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Ramakrishnan 44 combination o f variables is also established by the analysis o f variance. Experiment No. 7 showed a decrease in the value of microhardness after annealing. This was the worst combination and clearly shows that the mechanical alloying process was not carried out normally. 4.

The combination o f all three (higher values) primary factors gave a very high average effect o f -160.15 units. This negative interaction is clearly evident in the low microhardness value o f Sample No. 8 in both the replicates.

5.

Replicates 1 and 2 displayed almost similar trend in the results as enumerated above. This is also supplemented by the fact that three o f the samples: Sample Nos. 6, 5 and 3 and one bad sample: Sample 8, have the same respective combination of milling factors in both the replicates (after annealing).

All the above conclusions are based on average microhardness values o f annealed compacts subjected to MA and ECAE processes. 6.

The grain size values measured in all the available samples for the both the replicates establish the fact that, compacts with nanocrystalline grain sizes can be successfully produced by employing mechanical alloying and equal channel angular extrusion processes, under the experimental conditions investigated.

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Ramakrishnan 45 CHAPTER 6 Suggestions for Future Research During this work an attempt was made to better understand the benefits o f coupling mechanical alloying process with the equal channel angular extrusion process. The results obtained have shed some light on these issues, but it is recommended to consider the effects o f ECAE variables (number o f passes, extrusion temperature, route o f pass: A, B, C, and direction o f cutting the samples: along, transverse, etc.) on the properties o f the compacts in addition to the MA variables. This may lead to better understand the combination o f the variables to optimize both the grain size and the microhardness o f the compacts. X-ray diffraction results obtained for milled powder at various stages during the experiments and then analyzed to see if there is any substantial grain growth after ECAE will be a very useful addition to the work. The development o f a mathematical/computer model to simulate the MA and ECAE processes can be o f great interest to optimize the variables involved. In the present case, the model should incorporate the variables involved in MA and ECAE procedure. It should be noted however, that modeling is complicated because, o f the need for physical and thermodynamic data both at the nano­ scale and macro-scale levels.

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Ramakrishnan 46 REFERENCES 1

L. J. Prakash, “Application o f Fine Grained Tungsten Carbide Based Cemented Carbides,” Int. J. o f Refractory Metals & Hard Materials 13 (1995): 257.

2

T. Fischer, “Applying Nanostructured Materials to Future Gas Turbine Engines,” Journal o f Metals (May 1994): 32.

3

Xhemal Kaculi, “Effect o f Mechanical Alloying and Bulk Shear Processing on the Quality o f Tungsten Carbide Rods.” Masters thesis, Lamar University, 1999.

4

R. B. Schwarz and P. Nash, “Using Amorphous Phases in the Design of Structured Alloys.” Journal o f Metals (Jan. 1989): 27.

5

Anastasios Parasiris, “Consolidation o f WC-Co bv Simple Shear.” Masters thesis, Texas A&M University, 1999.

6

B. H. Kear and L. E. McCandish, “Chemical Processing and Properties o f Nanostructured WC-Co Materials,” Nanostructured Materials 3 (1993): 19.

7

R. B. Schwarz and P. Nash, “Using Amorphous Phases in the Design o f Structural Alloys,” Journal O f Metals (Jan. 1989): 27-31.

8

J. S. Benjamin, “Fundamentals o f Mechanical Alloying,” Materials Science Forum (1992): 88-90, 1-4.

9

J. S Benjamin, and T. S. Volin, ‘T he Mechanism o f Mechanical Alloying,” Metallureic Transactions 5 (1974): 1929-1930.

10

R. Sundaresan and F. H. Frees, “Mechanical Alloying,” Journal o f Metals (Aug. 1987): 22.

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Ramakrishnan 47 11

J. S. Benjamin, “Mechanical Alloying,” Scientific American 234, no. 5 (1976): 44.

12

H. Bakker and L. M. Di, “Atomic Disorder and Phase Transitions in Intermetallic Compounds by High-Energy Ball Milling,” Materials Science Forum (1992): 88-90,27.

13

Stephane M. Ferrasse, Vladimir K. Segal, K. T. Hartwig, and R. E. Goforth, “Microstructure and Properties o f Copper and Aluminum Alloy 3003 Heavily Worked by Equal Channel Angular Extrusion,” Metallurgical and Materials Transactions 28A. (April 1997): 1047-1048.

14

Anastasios Parasiris. “Consolidation o f WC-Co bv Simple Shear” Masters thesis, Texas A&M University, Aug. 1999.

15

G .E . Korth and R. L. Williamson, “Dynamic Consolidation o f Metastable Nanocrystalline Powders,” Metallurgical and Materials Transactions 26A, (Oct. 1995): 2571.

16

R. W. Siegel, “Nanostructured Materials,” http://www.nanophase.com/HTML/NANOMAT/siegel 11 .html 1-3.

17

R. W. Siegel, “What Is So Special About Nanostructured Materials and Coatings?” http://www.nanophase.com/HTML/NANOMAT/siegel2.html 4.

18

H. J. Cui, R. E. Goforth and K.T. Hartwig, “The Three-Dimensional Simulation o f Flow Pattern in Equal-Channel Angular Extrusion,” Journal of Metals (Aug. 1998): 23.

19

D. R. Maurice, and T. H. Courtney, “Modeling o f Mechanical Alloying: Part I, deformation, coalescence and fragmentation mechanisms,” Metallurgical

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Ramakrishnan 48 Transactions 2SA no. 1,(1994): 147-158. 20

T. H. Courtney, “M odeling o f Mechanical Milling and Mechanical Alloying,” Reviews in Particulate Materials 2, (1994): 63-116.

21

D. R. Maurice and T. H. Courtney, "Modeling o f the MA Process,” The Journal of Minerals. Metals & Materials Society 44 no. 8,(1992): 10-14.

22

Xhemal Kaculi, “Effect o f Mechanical Alloying and Bulk Shear Processing on the Quality o f Tungsten Carbide Rods.” Masters thesis, Lamar University, 1999.

23

D. C. Montgomery, Design and Analysis o f Experiments. 3rd ed. (New York: John Wiley & Sons 1991): 270.

24

W. G. Cochran and G. M. Cox, Experimental Design. 2nd ed. (New York: John Wiley & Sons 1950): 148-182.

25

Metal Test Methods and Analytical Procedures, “Standard Practice for Preparation o f Metallographic Specimens.” Annual Book o f ASTM Standards, 03.01, (1999): E 3 - 9 8 ,1 - 8 .

26

Metal Test Methods and Analytical Procedures, “Standard Test Method for Microhardness o f Materials.” ASTM Standards, 03.01, (1999): E 384 - 89,400.

27

E. F. Kaelble and S. F. Bartram, “Crvstallite-Size Determination from Line Broadening and Spotty Patterns.” Handbook o f X-Ravs. 1 7 .1 -1 7 .

28

M. Magini and A. Iasonna. “Experimental Supports to the Energy Transfer Collision Model in the Mechanical Alloying Process.” M aterial Science Forum. (1996): 225-227,229-236

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Ramakrishnan 49 APPENDIX A

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Ramakrishnan SO Table 7 Replicate 1: Vickers Microhardness of extruded WC-Co samples at different points of the material before annealing

Points

Exp. 1

Exp. 2

Exp. 3

Exp. 4

Exp. 5

Exp. 6

Exp. 7

Exp. 8

Point 1

480

362

402

232

234

446

509

387

Point 2

540

379

619

243

211

470

554

339

Point 3

240

473

576

190

206

490

665

306

Point 4

563

190

397

172

165

423

518

204

Point 5

347

152

261

208

319

546

680

195

Point 6

639

299

302

219

262

556

744

212

Point 7

483

383

263

277

418

540

445

185

Point 8

595

294

396

279

337

-

-

331

Point 9

290

210

258

289

190

-

-

270

Point 10

359

452

386

245

218

-

-

246

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Ramakrishnan SI Table 8 Replicate 1: Vickers microhardness of extruded WC-Co samples at different points of the material after annealing

Points

Exp. 1

Exp. 2

Exp. 3

Exp. 4

Exp. 5

Exp. 6

Exp. 7

Exp. 8

Point 1

614

750

898

550

899

1822

477

830

Point 2

646

557

737

853

2060

1544

489

853

Point 3

547

573

734

735

1290

977

399

575

Point 4

452

567

884

340

1158

1723

353

600

Point 5

578

570

715

430

1851

1170

452

662

Point 6

401

800

486

487

767

617

305

338

Point 7

666

820

474

321

769

724

275

310

Point 8

492

760

533

342

796

667

382

461

Point 9

530

1153

1287

1171

1108

960

934

544

Point 10

1030

973

1766

2145

921

1158

1103

430

Point 11

1430

1117

533

699

697

1513

679

558

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Ramakrishnan 52 Table 9 Replicate 2: Vickers microhardness of extruded WC-Co samples at different points o f the material before annealing

Points

Exp. 1

Exp. 2

Exp. 3

Exp. 4

Exp. 5

Exp. 6

Exp. 7

Exp. 8

1

696.4

232.5

667.7

681.8

321.5

1097.7

791.6

399.3

2

711.5

232.3

437.5

494.3

411.2

1176.5

580.9

454.8

3

290.8

331.4

564.8

541.8

368.4

620.9

653.9

350.8

4

475.5

408.2

531.8

539.1

238.9

903.6

549.3

460.6

5

672.8

265.3

447.1

447.8

440.2

477.0

212.1

457.7

6

584.0

249.0

511.7

440.2

357.7

791.6

846.6

231.0

7

365.8

349.9

337.3

509.1

603.1

850.2

443.7

397.5

8

308.3

175.1

453.4

498.3

207.2

846.6

586.1

487.1

9

525.5

289.4

432.2

470.9

380.6

815.0

722.8

438.2

10

413.1

301.7

585.0

490.3

196.9

799.8

317.7

358.2

11

400.4

270.1

543.7

535.4

224.2

940.6

625.4

632.4

12

443.7

248.4

446.4

433.5

689.7

623.1

456.3

13

459.1

339.1

878.2

677.9

538.2

14

526.4

284.0

788.3

193.7

399.3

15

541.8

319.8

668.9

651.5

487.1

16

741.7

17

1000.5

18

1061.3

19

989.0

20

1048.7

21

982.2

22

1009.9

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467.2

Ramakrishnan S3 Table 10 Replicate 2: Vickers microhardness o f extruded WC-Co samples at different points o f the material after annealing

Points

Exp. 1

Exp. 2

Exp. 3

Exp.4

Exp. 5

Exp. 6

Exp. 7

Exp. 8

1

571.7

340.5

473.9

372.0

895.7

839.4

544.6

462.8

2

728.5

652.7

490.3

526.4

727.1

1103.1

441.6

257.9

3

747.7

280.9

521.1

569.7

591.0

1019.4

332.3

490.3

4

329.9

443.6

455.6

413.7

649.3

1411.1

321.9

405.2

5

546.5

614.6

844.8

303.6

465.1

1024.2

440.0

300.1

6

597.7

339.0

427.6

673.6

693.5

1116.7

339.6

585.3

7

364.2

383.7

583.5

553.9

789.9

1369.3

635.1

469.1

8

520.9

319.3

512.3

633.8

817.0

780.2

597.2

611.5

9

412.5

298.8

764.0

689.5

756.3

1014.6

409.8

520.1

10

619.1

403.2

548.4

487.2

836.9

758.3

495.2

569.3

11

442.0

326.7

398.3

590.4

545.2

947.0

508.9

402.0

12

650.8

590.3

795.9

721.4

13

522.9

14

872.5

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Ramakrishnan 54 Table 11 Best/Worst samples: Replicate 1 and 2 (before annealing)

The samples have been sorted and arranged in the order: best to worst. The qualities o f the samples are based on the values o f microhardness.

Replicate No. 1 Before Annealing

Sample Nos.

Sample Nos.

Replicate No. 2 Before Annealing Time

Speed

Ball/ Pow.

6

40

150

30:1

30:1

7

10

550

30:1

150

10:1

4

10

150

30:1

10

550

10:1

3

10

550

10:1

2

40

150

10:1

1

10

150

10:1

8

40

550

30:1

8

40

550

30:1

5

40

550

10:1

5

40

550

10:1

4

10

150

30:1

2

40

150

10:1

Time

Speed

Ball/Pow.

7

10

550

30:1

6

40

150

1

10

3

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Ramakrishnan SS Table 12 Best/Worst samples: Replicate 1 and 2 (after annealing)

The samples have been sorted and arranged in the order: best to worst. The quality o f the samples is based on the average values o f microhardness.

Replicate No. 1 After Annealing

Sample Nos.

Replicate No. 2 After Annealing

Sample Nos.

Time

Speed

Ball: Pow.

6

40

150

30:1

10:1

5

40

550

10:1

550

10:1

3

10

550

10:1

40

150

10:1

1

10

150

10:1

4

10

150

30:1

4

10

150

30:1

1

10

150

10:1

8

40

550

30:1

8

40

550

30:1

2

40

150

10:1

7

10

550

30:1

7

10

550

30:1

Time

Speed

Ball.Pow.

6

40

150

30:1

5

40

550

3

10

2

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Ramakrishnan 56 Table 13 Effects o f milling variables before annealing (using average microhardness values)

Replicate No. 1

Replicate No. 2

Average, of Replicate No. 1 & N o.2 (or number of replicates n = 2)

Milling time,

A = -34.54

Milling time,

Milling speed, B = -2.075

Milling speed,

B = -26.38

Milling speed, B = -14.23

Ball/Pow. ratio, C = 42.52

Ball/Pow. ratio, C = 194.19

Ball/Pow. ratio, C = 118.36

Time & speed, AB = - 106.17

Time & speed, AB = -124.97

Time and ball/pow. ratio, AC = 51.43

Time and ball/pow. ratio, AC = 150.315

Time and ball/pow. ratio, AC = 100.87

Speed and ball/pow. ratio, BC = 63.43

Speed and ball/pow. ratio, BC = -111.56

All 3 factors,

All 3 factors,

Milling time,

A = -80.675

Time & speed, AB = -143.78

ABC = -145.88

ABC = -136.22

A = -57.61

Speed and ball/pow. ratio, BC = -24.07 All 3 factors, ABC = -141.05

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Ramakrishnan 57 Table 14 Modeling results

Location Within Attritor

Limited Diffusion

Impact and Mixing

Bottom, near middle

Bottom, high shear zone

Powder dead zone

Power Involved P (Watts) x 103

Kinetic Energy E (Joules) x 10'3

Speed (rpm)

250

0.00658

0.0103

500

0.0806

0.208

250

0.0148

0.0087

500

0.133

0.313

250

0.0

0.0

500

0.0016

0.00044

250

0.00658

0.0059

500

0.0148

0.00157

250

0.0

0.0

500

0.0

0.0

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Ramakrishnan 58 Table 15 Analysis o f Variance (ANOVA) o f microhardness values after annealing

Source o f V ariation

Sum of S quares

Degrees of Freedom

M ean S quares

F = M ean Sq./M ean Sq. of E

Milling Time (A)

23914

1

95654

4.54**

Milling Speed (B)

5441

1

21762

1.03

B/P Ratio (C)

645

1

2579

0.12

AB

2040

1

8161

0.39

AC

3993

1

15972

0.76

BC

66667

1

266669

12.64*

ABC

25648

1

102592

4.86**

Error (E)

168727

8

21091

Total

682116

15

* **

Significant at 95% and 99% confidence levels. Significant at only 90% confidence level.

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Ramakrishnan 59 APPENDIX B

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Ramakrishnan 60

in co

CO

CO

CO

o

o

sjunoQ

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Intensity vs. Two Theta Plot for Replicate 1 - Sample 1

Replicate 1-Sample 1

CO

CO

CO

m co

o o

o

co

o o

s)un 3

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Intensity vs. Two Theta Plot for Replicate 1 - Sample 2

R1-Sample 2

Ramakrishnan 61

Ramakrishnan 62

in co co

R1-Sample 3

CO

o o

CO

o o

o

o

o

o

to

sju n o o

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Intensity vs. Two Theta Plot for Replicate 1 - Sample 3

in co

CO

co

CO

R2-Sample 3

CO

o o

in 04

CO

o o o

o

o

o

sjunoQ

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 3

Ramakrishnan 68

Ramakrishnan 69

R2-Sample 4

CO

CO

CO

o

o

CO

o

o

o o

o CM

siu n o o

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 4

in co

Ramakrishnan 70

in

m co

CO

R2-Sample 5

CO

CO

CO

CO

o

o

m

CO

o o co

s)u n o 3

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 5

in

Ramakrishnan 71

m co

R2-Sample 6

CO

CM

CO

o o

CO

o

o

o

o

sjunoQ

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 6

m

CM

o o

o

o o

CO

o

o o

o

sju n o o

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 7

R2-Sample 7

Ramakrishnan 72

Ramakrishnan 73

to

in

00

R 2-Sam ple

8

CO

CO

o

CO

o

CO

o o

o o o

o o

o o

o

o

CM

sjunoo

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Intensity vs. Two Theta Plot for Replicate 2 - Sample 8

CO

Ramakrishnan 74 APPENDIX C

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Ramakrishnan 75

Figure 4.15 SEM image o f Sample 7 - Replicate 2 at lOOx magnification

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Ramakrishnan 76

Figure 4.16 SEM image o f Sample 7 - Replicate 2 at l,000x magnification

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Ramakrishnan 77

Figure 4.17 SEM image o f Sample 7 - Replicate 2 at 10,000x magnification

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Ramakrishnan 78

Figure 4.18 SEM image o f Sample 7 - Replicate 2 at 25,000x magnification

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Ramakrishnan 79

3.•'"it5526* fc g «

Figure 4.19 SEM image o f Sample 7 - Replicate 2 at 50,000x magnification

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

Replicate 2: All 8 samples after annealing

80

♦ 70

60

E

c

0>

ac * 2 O

30

20

10

0

200

400

600

800

Average Microhardness (HV) Figure 4.20 Plot of Average Microhardess vs. Grain Size

1000

1200

Ramakrishnan 81 BIOGRAPHICAL NOTE Kannan Ramakrishnan was bom on May 22,1973 in Kerala, India. He obtained his Bachelor o f Engineering degree in Mechanical Engineering from Walchand Institute of Technology-Shivaji University, Solapur, India on July 1994. He was selected for employment as a graduate trainee engineer in Walchandnagar Industries Ltd., Pune, India on August 1994. After successfully completing the training, he was absorbed as a Purchase Engineer in the same company on February 1996. Kannan enrolled for the Masters o f Engineering Science program in the Department of Mechanical Engineering, at Lamar University, Beaumont in August 1996 and earned his Masters Degree in August 1998. He also enrolled for the Doctoral Degree Program in the Mechanical Engineering Department, at Lamar University, Beaumont in June 1998. This dissertation is submitted in partial fulfillment o f the requirements for the doctoral degree.

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